The geometries in which high-performance superconducting oxide composites may be successfully fabricated are constrained by the necessity of texturing the material to achieve adequate critical current density. The current-carrying capacity of a superconducting oxide composite depends significantly on the degree of crystallographic alignment and intergrain bonding of the oxide grains, together known as "texturing", induced during the composite manufacturing operation.
Known processing methods for texturing superconducting oxide composite articles include various forms of heat treatment as well as longitudinal deformation. Certain superconducting oxide grains can be oriented along the direction of an applied strain, a phenomenon known as deformation-induced texturing (DIT). Longitudinal deformation techniques like pressing, drawing and rolling have been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. Heat treatment under conditions which at least partially-melt and regrow desired superconducting phases may promote texturing by enhancing the anisotropic growth of the superconducting grains, a phenomenon known as reaction-induced texturing (RIT).
However, not all texturing methods are equally applicable to, or effective for, all superconducting oxides. Most of these materials have very few known effective texturing mechanisms. For example, known techniques for texturing the two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors (BSCCO 2212 and BSCCO 2223, respectively) are described in Tenbrink, Wilhelm, Heine and Krauth, Development of Technical High-Tc Superconductor Wires and Tapes, Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992), and Motowidlo, Galinski, Hoehn, Jr. and Haldar, Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting, Apr. 12-15, 1993. Techniques for manufacturing multifilamentary articles with sufficient texturing to provide acceptable critical current densities from BSCCO 2223 are presently limited to the production of highly aspected forms such as tapes.
The effectiveness of a particular DIT technique will depend on how closely the applied strain vectors correspond to the slip planes in the superconducting oxide. Thus, superconducting oxides such as the BSCCO family, which have a micaceous structure characterized by highly anisotropic preferred cleavage planes and slip systems, are known to be most effectively DIT textured by non-axisymmetric techniques such as pressing and rolling, which create highly aspected (greater than about 5:1) forms. For perovskite structures like the 123 phase of the yttrium-barium-copper-oxide (YBCO) family, which lack preferred cleavage planes and slip systems, longitudinal deformation is generally less effective in improving critical current density and the differences in texturing obtainable by axisymmetric and non-axisymmetric techniques are less pronounced.
Materials which exhibit peritectic melting can be effectively textured in a variety of geometries by melt textured growth, an RIT technique. Peritectic decomposition and the reformation of the oxide superconductor from the liquid+(other) solid phase is the basis for melt textured growth of the two-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors (BSCCO-2212) in round wire and tape forms, as described, for example, in Kase et al. IEEE Transmag 27(2), 1254 (March 1991). Because 2212 totally melts and reforms during melt-textured growth, the texturing induced by deformation prior to the melting will not influence the final structure.
However, some of the most promising superconducting oxides, such as BSCCO 2223, cannot be effectively textured by the melt-textured growth technique. Instead of peritectic melting, BSCCO 2223 exhibits irreversible melting in that solid 2223 does not form directly from a liquid of 2223 composition. RIT techniques applicable to BSCCO 2223 have been described, for example in U.S. patent application Ser. No. 08/041,822 filed Apr. 1, 1993, entitled IMPROVED PROCESSING OF OXIDE SUPERCONDUCTORS, and U.S. Ser. No. 08/198,912 filed Feb. 17, 1994, entitled IMPROVED PROCESSING OF OXIDE SUPERCONDUCTORS. The basis of such techniques is some type of partial melting, such as eutectic melting, solid solution melting or formation of non-equilibrium liquids, in which the oxide superconductor coexists with a liquid phase rather than being totally decomposed. However, such techniques are inherently more dependent on the geometry of the initial phase than melt-textured growth, and texturing induced by deformation prior to the partial melting will have a significant impact on the texturing of the final product. The RIT technique described in U.S. patent application Ser. No. 08/041,822 cited above, for example, has been observed to provide the greatest improvement in the J.sub.c 's of BSCCO 2223 samples when used in combination with a highly non-axisymmetric DIT technique, rolling. In short, superconducting oxides with irreversible melting characteristics such as BSCCO 2223 can be adequately textured by known techniques in highly aspected forms such as tapes, but scalable methods for manufacturing round wires and other low aspect ratio composite multifilamentary articles with sufficient texturing to provide acceptable critical current densities are not presently available.
This limitation has considerable significance. Many of the superconductor applications that have the greatest potential for energy conservation involve operating the superconductor in the presence of an AC magnetic field, or require that the superconductor carry an AC current. In the presence of time-varying magnetic fields or currents, there are a variety of mechanisms that give rise to energy dissipation, hereafter called AC losses, even in superconductors. Thus, the superconductor geometry must be selected to reduce AC losses, in order to preserve the intrinsic advantage of superconductors, the absence of DC electrical resistance. The physics governing AC losses in low temperature superconducting composite materials have been described and analyzed, C.F., Wilson, Superconducting Magnets, Ch 8 (1983, 1990), and round, multifilamentary composite geometries with twisted, low aspect ratio superconducting filaments have been demonstrated to have significantly better AC loss characteristics than highly aspected, untwisted or monofilamentary forms. To minimize hysteretic losses, the superconductor must preferably be subdivided into many small filaments that are dimensionally uniform and discrete along the length of the conductor. Low aspect ratio filaments (about 4:1 or less) will have lower hysteretic losses in all but unidirectional magnetic fields, so these filament dimensions are generally preferred. To minimize eddy current losses, the matrix resistivity must preferably be increased and the twist pitch of the filaments must preferably be tightened, i.e., reduced.